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A common-envelope phase is a brief, unstable episode in a binary system: an evolved star expands and engulfs its companion, leaving both stellar cores orbiting inside one shared envelope of gas. Drag pulls the cores into a tighter orbit and transfers energy to the envelope. The gas may be expelled, leaving a close binary, or the cores may merge.
How does a common-envelope phase begin?
As a star evolves, it can expand enough to engulf its companion. The companion and the evolved star’s core then move through the same extended envelope. This is different from ordinary, stable mass transfer between two stars: the interaction is rapid and involves both cores inside the donor star’s gas.
Why does the orbit shrink?
As the cores move through the gas, hydrodynamic drag and gravitational interactions take energy and angular momentum from their orbit. The orbit contracts, and some of the released orbital energy is deposited in the envelope. That energy can make the gas expand and help it escape the system.
How astronomers describe the energy balance
A standard model compares the energy needed to unbind the envelope, called its binding energy, with the change in orbital energy as the binary tightens. It uses an efficiency parameter, αCE, to represent the fraction of available orbital energy that helps eject the gas. The framework is useful for describing the process, but it is not a settled recipe for predicting the outcome: the envelope’s binding energy depends on the adopted stellar structure and the boundary chosen between the core and envelope, while the efficiency is difficult to determine reliably.
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Can the stars survive?
There are two broad outcomes. If the envelope is expelled before the cores collide, the result is a close post-common-envelope binary. If the orbit keeps decaying and the envelope is not successfully ejected, the cores can merge. Which outcome occurs depends on the binary’s properties, the structure of the evolved star, and the still-uncertain physics of the interaction.
Why does this phase matter for binary-star evolution?
Common-envelope evolution offers a way for a widely separated pair to become a close binary containing compact objects. The National Academies’ decadal survey identifies uncertainties in this phase as relevant to whether some systems can form Type Ia supernova progenitors and whether related evolutionary pathways produce AM CVn stars, supersoft X-ray sources, or double white dwarfs. These are possible outcomes of particular evolutionary pathways, not products of every common-envelope event. National Academies, Pathways to Discovery in Astronomy and Astrophysics for the 2020s.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What remains uncertain?
The shared-envelope picture, orbital tightening, and the possibility of either ejection or merger form the basic explanation. The detailed energy sources and sinks, the envelope’s binding energy, and the conditions required for successful ejection are harder to establish. Modern reviews discuss the challenges of modeling the event with multidimensional hydrodynamic simulations and calibrating the energy-efficiency parameter. Ivanova et al., “Common envelope evolution: where we stand and how we got there” (2013).
When evaluating a proposed model, the key questions are whether it fully unbinds the envelope, whether the cores survive, what final orbital separation it predicts, and what assumptions it makes about envelope binding energy and energy-transfer efficiency.
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